Mesothelial cells constitute the lining of the three serous sacs of the body i.e. the pleura, pericardium and peritoneum. The more common type of malignant neoplasia of the serous sacs is seeding by metastatic tumors and primary malignancy of the mesothelium is unusual. Of the three sacs, the pleura is the most common site of malignant mesothelioma. Involvement of the peritoneum is extremely rare.We report 3 cases of malignant mesothelioma of the peritoneum. All of them were female. Their ages were 57, 67 and 72 years, respectively. The patients presented with abdominal discomfort and/or ascites. The extent of the tumors ranged from a peritoneal implant to widespread infiltration of the peritoneum and omentum. Histologic examination in Case 1 showed the presence of a diffusely infiltrating papillary mesothelioma without a sarcomatoid component. A mesodermal element was present in the other two cases. In order to establish a morphological diagnosis of malignant mesothelioma, the possibility has to be excluded of a metastatic adenocarcinoma.